Method and system for distributing power to networked devices

ABSTRACT

One embodiment disclosed relates to a system for power distribution to network devices. The system includes a plurality of network switches each having an internal power supply and a plurality of ports for connecting to the network devices and an external power supply having a plurality of output ports for connecting to the network switches. The external power supply communicates power available to the network switches. Each network switch determines amounts and priority levels of power for the network devices connected thereto, sums together the amounts at each priority level, determines additional amounts and priority levels of power required beyond the is internal power supply capability, and sends a power request to the external power supply. The external power supply allocates power to the network switches depending on the power requests received.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to networking and communicationstechnology.

2. Description of the Background Art

Power over LAN™ or Power over Ethernet, is a new technology that enablesDC power to be supplied to Ethernet data terminals over ordinary localarea network (LAN) cabling such as Category 5 cabling. This technologyenables the terminals, termed powered devices (PDs), to receive theiroperating power over the same Ethernet LAN connection that they use fordata communication. It thus eliminates the need to connect each terminalto an AC power socket, and the need to provide each terminal with itsown AC/DC power converter. The technology also enables PDs to berecognized as such by a “signature” generated by the terminal. The LANMAN Standards Committee of the IEEE Computer Society is developingspecifications for Power over LAN systems, as described in IEEE DraftsP802.3af/D3.0 or later, entitled “Data Terminal Equipment (DTE) Powervia Media Dependent Interface (MDI)” (IEEE Standards Department,Piscataway, N.J., 2001), which is also incorporated herein by reference.The specifications are referred to herein as standard 802.3af.

A Power over LAN system comprises an Ethernet switch and a power hub,which serves as the DC power source, along with a number of PDterminals, which communicate via the switch and draw power from the hub.The system is typically connected in a star topology, with each terminallinked by a cable to the switch and hub. The power hub in one chassismay be integrated with the switch in a second chassis, in a consolecontaining both chassis, in what is known as an “end-span”configuration. Alternatively, the-power hub chassis may be locatedbetween the switch chassis and the terminals, in a “mid-span”configuration. DC power is carried to the loads (i.e., the terminals)over twisted pairs provided by Category 5 cabling. The end-spanconfiguration uses twisted-data-pairs that are also used for Ethernetcommunication; the mid-span configuration uses spare twisted-spare-pairsthat are not used for Ethernet communication.

SUMMARY

One embodiment of the invention pertains to a system for powerdistribution to network devices. The system includes a plurality ofnetwork switches each having an internal power supply and a plurality ofports for connecting to the network devices and an external power supplyhaving a plurality of output ports for connecting to the networkswitches. The external power supply communicates power available to thenetwork switches. Each network switch determines amounts and prioritylevels of power for the network devices connected thereto, sums togetherthe amounts at each priority level, determines additional amounts andpriority levels of power required beyond the internal power supplycapability, and sends a power request to the external power supply. Theexternal power supply allocates power to the network switches dependingon the power requests received.

Another embodiment of the invention pertains to a method of powerdistribution to network devices. Amounts and priority levels of powerare determined for the network devices connected to each powerdistributor of a plurality of power distributors. The amounts at eachpriority level are summed together at each power distributor, and adetermination is made of additional amounts and priority levels of powerrequired beyond an internal power supply capability of each powerdistributor.

In another embodiment, the method of distributing power to networkdevices maintaining in the switch a table of the amount and prioritylevel for each switch port. The table is used to allocate availablepower to higher priority devices when insufficient power is available tofully power all of the connected devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram depicting a conventional system fordistributing power.

FIG. 2 is a block diagram depicting a system for distributing power inaccordance with an embodiment of the invention.

FIG. 3 is a diagram of an intelligent power supply in accordance with anembodiment of the invention.

FIG. 4 is a diagram of an intelligent power distributor in accordancewith an embodiment of the invention.

FIG. 5 is a flow chart depicting a method of power authorization by anintelligent power supply in accordance with an embodiment of theinvention.

FIG. 6 is a flow chart depicting a method of generating power requestsby intelligent power distributors using arbitration in accordance withan embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 is a block diagram depicting a conventional system 100 fordistributing power. The conventional system 100 includes a conventionalexternal power supply 102, multiple conventional power distributors 104coupled to the power supply 102, and multiple power user devices 106coupled to each power distributor 102. The external power supply 102 andthe power distributors 104 are connected by way of a conventional cableassembly 108.

The conventional power supply 102 typically distributes an equal amountof power to each power distributor 104 (load) connected thereto. Thismay be accomplished by either current sense and sharing, or by separatesupplies within the box. The conventional cable assembly 108 typicallyprovides power and return wires. For power-sharing purposes, asense-signal line may be provided. The sense signal may comprise alow-current analog signal generated using low-impedance circuitry. Thesense signal is an indication of the load due to the associated powerdistributor 104.

Similarly, the conventional power distributor 104 typically distributesan equal amount of power to each power user device 106 (load) connectedthereto. Again, this may be accomplished by either current sense andsharing, or by separate supplies within the box. The power user devices106 comprise loads which “demand” power from the power distributors 104without any intelligent prioritization.

The conventional external supply 102 has a mechanism to protect itselfif it is overloaded. If the load is too high, then the power supply 102may “crowbar” to avoid damage from being overloaded. A crowbar circuitis an overvoltage protection mechanism which, when a voltage limit isexceeded, may shunt a low-resistance across the power supply outputterminals.

In contrast to the conventional system, an embodiment of the presentinvention comprises a novel power distribution system where a protocolallows the recipients of the power to indicate their requested amountsof power and priority levels thereof. The power source evaluates thepower demand from the multiple recipients and balances the power demandsagainst the available power so as to optimize the power distribution ona prioritized basis. The algorithm used by the power source is such thata device deemed “higher priority” will be given power over anotherdevice that is defined as a “lower priority.” The power source repliesback with an indication of allowed power usage to each recipient.

One embodiment of the invention provides a unique solution to a generalproblem in the “Power over Ethernet” (PoE) technology space. PoE systemsare required to provide up to 15 watts per port even though many devicesdo not require that amount of power. For example, a 48 port switch underPoE would conventionally provide 15 watts per port so would need a 720watt power supply. The cost of such a 720 watt supply and the packaging,air flow, and so on, to support that large of a supply is quiteexpensive. This problem is advantageously overcome in an intelligent wayusing embodiments of the present invention so as to provide powerflexibly with an under-provisioned power system (i.e. a system withoutfull capacity to guarantee full power to all ports at all times.)

For example, one embodiment of the present invention comprises multipleintelligent PoE switches to be connected to a single intelligentexternal power supply (EPS), where the total power capacity of theswitches and EPS is less than the theoretical maximum required (i.e.less than 15 watts times the number of ports for user devices). Thesystem of the present invention intelligently allocates the availablepower capacity of the EPS to the various switches connected thereto. Forinstance, suppose a first switch wants 250 watts of power from the EPSand a second switch only needs 50 watts of power from the EPS. Power canbe requested for each switch up to a point that would exceed the maximumtotal supply of the EPS, then the algorithm limits the allocated powerso that the EPS capacity is not exceeded. In addition, the priority ofpower being distributed may be determined and utilized. For example,suppose both switches were drawing power defined as low priority. If thesecond switch were to suddenly need 100 watts of high priority power,the system would demand that the first switch reduce its powerconsumption as necessary to allow the second switch to take the neededhigh priority power.

FIG. 2 is a block diagram depicting a system 200 for distributing powerin accordance with an embodiment of the invention. The components andinteractions in this system 200 differ substantially from those in theconventional system 100 of FIG. 1. The system 200 of FIG. 2 includes anintelligent external power supply (EPS) 202, multiple intelligent powerdistributors (PDs) 204 coupled to the intelligent EPS 202, and multipleprioritized power user devices 206 coupled to each intelligent PD 202.The intelligent EPS 202 and intelligent PDs 204 are connected by way ofa cable assembly 208 having a digital communications interface. Thedigital communications interface may comprise, in one embodiment, alow-frequency serial interface.

One embodiment of the intelligent EPS 202 is described further below inrelation to FIG. 3. In general, the intelligent EPS 202 may beconfigured to provide power depending on requested quantities of power,where the power requests are received from the intelligent powerdistributors 204 by way of the aforementioned digital communicationsinterface. The intelligent EPS 202 may also be configured with thecapability to detect whether the load from an intelligent PD 204 isexceeding an authorized amount for that distributor.

One embodiment of an intelligent PD 204 is described further below inrelation to FIG. 4. In general, the intelligent PD 204 may be configuredto determine amounts and prioritization of power requested by (orassigned to) the power user devices 206.

FIG. 3 is a diagram of an intelligent power supply 202 in accordancewith an embodiment of the invention. The intelligent power supply 202comprises an external power supply to the intelligent power distributors204. The intelligent external power supply (EPS) 202 includes a powersupply 302, a shared power bus 304, a EPS controller 306, a EPS controlbus 308, EPS programmable current sense and control units 310, and acommunications channel 312, and a cable assembly interface 314.

The power supply 302 of the intelligent power supply 202 may beconfigured so as to be optimized for distribution within a low formfactor and a low cost. In other words, the internal power supply 302need not be capable of simultaneously providing full power to all userdevices 206 connected via the intelligent power distributors 204 to theintelligent power supply 202.

The shared power bus 304 couples the power supply 302 to each of the EPSprogrammable current sense and control units 310. Each EPS programmablecurrent sense and control unit 310 couples to an output port comprisinga cable assembly interface 314. Each cable assembly interface 314connects to a corresponding cable assembly with communications interface208.

Each EPS programmable current sense and control unit 310 senses currentto the corresponding output port and includes a switch to open or closethe electrical connection from the shared power bus 304 to thecorresponding output port. The current sensing may be performed bymeasuring a voltage across a low resistance element, and the switch maycomprise, for example, a field effect transistor (FET) switch.

The EPS controller 306 is coupled via the EPS control bus 308 to each ofthe EPS programmable current sense and control units 310. The EPScontroller 306 comprises a processor and associated memory and is usedto control the EPS programmable current sense and control units 310 andother components (such as a fan and so on). The memory may include anexternal power supply table (EPS table) 316 that includes, for example,amounts and priority levels of power for each output port of theintelligent power supply 202.

The EPS controller 306 is also coupled to each of the output ports byway of a communications channel 312. For example, the communicationschannel 312 may comprise a serial communications channel or otherdigital communications channel. The communications channel 312 may beused, for example, to receive power requests from and power allocationsto the intelligent power distributors 204.

In accordance with an embodiment of the invention, if a powerdistributor 204 coupled to one of the output ports draws more currentthan it is allocated or authorized to draw, then the correspondingprogrammable current sense and control unit 310 may be utilized toswitch off power to that power distributor 204.

FIG. 4 is a diagram of an intelligent power distributor (PD) 204 inaccordance with an embodiment of the invention. The intelligent PD 204includes a cable assembly interface 402, a power bus 403, a powermultiplexer 404, an internal power supply 406, PD programmable currentsense and control units 408, a PD controller 410, a PD control bus 412,and LAN ports 414. In accordance with one embodiment of the invention,the power distributor 204 may comprise a network local area network(LAN) switch.

The internal power supply 406 within an intelligent power distributors204 may be configured so as to be optimized for distribution within alow form factor and a low cost. In other words, the internal powersupply 406 need not be capable of providing full power to all userdevices 206 simultaneously. For example, an intelligent powerdistributor 202 may include, for example, 200 “Power over Ethernet”ports, each of which may provide up to 15 watts to a user device 206. Inaccordance with an embodiment of the invention, the intelligent powerdistributor 202 may be configured with an internal power supply with acapacity that is substantially less than 200×15=3,000 watts.

The cable assembly interface 402 of the intelligent power distributor204 couples via a cable assembly with communications interface 208 to acorresponding cable assembly interface 314 of the intelligent powersupply 202. The power lines from the cable assembly are coupled via thepower bus 403 to an input of the power multiplexer (mux) 404. Theinternal power supply 406 is also coupled to an input of the power mux404.

The power mux 404 is controlled by the PD controller 410 via the PDcontrol bus 412. In accordance with one embodiment of the invention, thePD mux 404 is controllable so as to switch available power (from thepower bus 403 and/or the internal power supply 406) to the ports 414 ofthe intelligent PD 204. For example, the intelligent PD 204 may comprisea network switch with two banks (left bank and right bank) of twelve LANports 414 each. In one specific implementation, the power mux 404 may becontrollable so as to switch power from the internal power supply 406 tothe left bank, to the right bank, or to both banks of ports. Similarly,the power mux 404 may be controllable so as to switch power from theexternal power supply 202 (via the power bus 403) to the left bank, tothe right bank, or to both banks of ports. Of course, in otherimplementations, more than two banks may be used, and the banks may haveother numbers of ports.

Each PD programmable current sense and control unit 408 senses currentto the corresponding port 414 and includes a switch to open or close theelectrical connection from the power mux 404 to the corresponding port414. The current sensing may be performed by measuring a voltage acrossa low resistance element, and the switch may comprise, for example, afield effect transistor (FET) switch.

The PD controller 410 is coupled via the PD control bus 412 to each ofthe PD programmable current sense and control units 408. The PDcontroller 410 comprises a processor and associated memory and is usedto control the PD programmable current sense and control units 408, thepower mux 404, and other components (such as a fan and so on). Thememory may include a power distributor table (PD table) 420 thatincludes, for example, amounts and priority levels of power for eachport 414 of the intelligent PD 204.

In accordance with an embodiment of the invention, the priority level ofpower for each port 414 is dependent on the identity or type of networkdevice 206 connected to the port 414. For example, higher prioritydevices may include IP-enabled telephone devices. As another example,higher priority devices may include wireless access ports. In accordancewith another embodiment, the priority levels of power for the ports 414may be configurable into the PD table by a network administrator.

The PD controller 410 is also coupled to each of the cable assemblyinterface by way of a communications bus 416. For example, thesecommunications may comprise serial communications or other digitalcommunications. The communications bus 416 may be used, for example, tosend power requests to and power allocations from the intelligent powersupply 202.

In accordance with an embodiment of the invention, if a user device 206coupled to one of the ports 414 draws more current than it is allocatedor authorized to draw, then the corresponding PD programmable currentsense and control unit 408 may be utilized to switch off power to thatuser device 206. In one implementation, capacitors 418 areadvantageously coupled to the power lines going to each port 414. Thesecapacitors 418 may be used to prevent a temporary surge of power drawnfrom one port 414 from adversely affecting the user devices 206 coupledto the other ports 414.

FIG. 5 is a flow chart depicting a method 500 of power authorization byan intelligent EPS 202 in accordance with an embodiment of theinvention. In a preliminary step, registers within the intelligent EPS202 may be initialized 502 to set the registers to their default values.

Serial (or other digital) communication between the intelligent EPS 202and the intelligent PDs 204 is then initiated 504. For example, theinitiation 504 of communication may be implemented by the intelligent PD204 pulsing an interrupt signal to indicate that it is ready tocommunicate, or by a handshake between each intelligent PD 204 and theintelligent EPS 202.

The intelligent EPS 202 calculates and communicates 506 the poweravailable per port to the intelligent PDs 204. The communication isperformed by way of the digital communication channel therebetween. Inone implementation, the calculation may comprise simply dividing a totalpower capacity of the EPS 202 by the number of output ports withintelligent PDs 204 connected thereto. For example, if the powercapacity of the EPS 202 is 600 watts, and four output ports have powerdistributors 204 connected thereto, then the power available per port iscalculated to be 600 watts/4 PDs=150 watts per PD. In an alternateembodiment, the total power available may be communicated instead of thepower available per port.

The intelligent EPS 202 then receives 508 power requests from theintelligent PDs 204. These power requests may include an amount of powerrequested at each priority level from each intelligent PD 204. Forexample, each intelligent PD 204 may determine amounts and prioritylevels of power for the network devices 206 connected thereto, sumtogether the amounts at each priority level, determine additionalamounts and priority levels of power required beyond the internal powersupply 406 capability, and send a power request with these additionalamounts and priority levels of power to the intelligent EPS 202.

In one embodiment, the intelligent PDs. 204 may advantageously arbitrateamongst themselves in the process of generating the power requests. Ahigh-level process for such arbitration is described below in relationto FIG. 6. In an alternate embodiment of the invention, the powerrequests may be generated individually by the intelligent PDs 204(without mutual interactive arbitration therebetween). In such anembodiment, the intelligent EPS 202 may be configured to apply analgorithm that allocates the available power to the higher priorityrequests without exceeding the total capacity of the EPS 202.

After receiving the power requests from all the intelligent PDs 204, theintelligent EPS 202 totals the requested amounts to check as to whetherthe total requested amount is within the total capacity of the EPS 202.If so, then the power requests are acknowledged 510 by the EPS 202sending acknowledgement messages to the PDs 204. The acknowledgementmessage indicates the power being authorized and allocated to each PD204. If the total capacity is exceeded by the total requested amount,then, in one embodiment, the intelligent EPS 202 applies an algorithmthat allocates the available power to the higher priority requestswithout exceeding the total capacity of the EPS 202.

The system 200 thus allocates power from the intelligent EPS 202 to theintelligent PDs 204. Subsequently, the system is configured to detect animbalance or port change. For example, an imbalance may occur if one PD204 begins to draw power beyond its authorized and allocated amount. Aport change refers to the addition or removal of a PD 204 from the portsof the EPS 202. If an imbalance or port change is detected, then themethod 500 loops back to the step where communication between the EPS202 and PDs 204 is initiated 504, and the method 500 continues on fromthere to re-allocate the available power.

FIG. 6 is a flow chart depicting a method 600 of generating powerrequests by intelligent power distributors 204 using arbitration inaccordance with an embodiment of the invention. In a preliminary step,registers within each intelligent PD 204 may be initialized 502 to setthe registers to their default values.

The intelligent PDs 204 await and look for a signal to arbitrate powerrequests. When such an arbitration signal is received 604, then theintelligent PDs 204 arbitrate power requests amongst themselves. Thearbitration messages may be communicated by way of the digitalcommunications channels linking the PDs 204 to the EPS 202, or they maybe communicated by way of separate communication channels (notillustrated) between the PDs 204.

In one embodiment, the arbitration 606 may be implemented using amaster-slave arbitration algorithm. In such an embodiment, one PD 204 isdesignated as the master, and the other PDs 204 are designated asslaves. Various master-slave arbitration algorithms are known to thoseof skill in the pertinent art, and it is contemplated that the scope ofthe invention should encompass various master-slave arbitrationalgorithms. In an alternate embodiment, the arbitration 606 may beimplemented using a peer-to-peer arbitration algorithm. In such anembodiment, none of the PDs 204 is designated as the master. Variouspeer-to-peer arbitration algorithms are known to those of skill in thepertinent art, and it is contemplated that the scope of the inventionshould encompass various peer-to-peer arbitration algorithms.

Once the arbitration 606 is complete, then the arbitrated power requestsare provided 608 from the intelligent PDs-204 to the intelligent EPS202. Specific registers may be used to provide the power requests.Thereafter, the process 600 may loop back to initialize 602 theregisters and await 604 another arbitration signal.

In one alternate embodiment of the invention, the intelligent EPS 202and the multiple intelligent PDs 204 may be integrated together in asingle power distribution unit or chassis. In this embodiment, the EPS202 would no longer-be “external”, rather it would be internal to theunit. The multiple PDs 204 would then comprise subunits of theintegrated power distribution unit. Such an integrated powerdistribution unit would be advantageously intelligent and flexible indistributing its available power on a prioritized basis.

In the above description, numerous specific details are given to providea thorough understanding of embodiments of the invention. However, theabove description of illustrated embodiments of the invention is notintended to be exhaustive or to limit the invention to the precise formsdisclosed. One skilled in the relevant art will recognize that theinvention can be practiced without one or more of the specific details,or with other methods, components, etc. In other instances, well-knownstructures or operations are not shown or described in detail to avoidobscuring aspects of the invention. While specific embodiments of, andexamples for, the invention are described herein for illustrativepurposes, various equivalent modifications are possible within the scopeof the invention, as those skilled in the relevant art will recognize.

These modifications can be made to the invention in light of the abovedetailed description. The terms used in the following claims should notbe construed to limit the invention to the specific embodimentsdisclosed in the specification and the claims. Rather, the scope of theinvention is to be determined by the following claims, which are to beconstrued in accordance with established doctrines of claiminterpretation.

1. A system for power distribution to network devices, the systemcomprising: a plurality of network switches each having an internalpower supply and a plurality of ports for connecting to the networkdevices; an external power supply having a plurality of output ports forconnecting to the network switches, wherein the external power supplycommunicates power available to the network switches, wherein eachnetwork switch determines amounts and priority levels of power for thenetwork devices connected thereto, sums together the amounts at eachpriority level, determines additional amounts and priority levels ofpower required beyond the internal power supply capability, and sends apower request to the external power supply, and wherein the externalpower supply allocates power to the network switches depending on thepower requests received.
 2. The system of claim 1, wherein cablingconnecting the external power supply and the network switches includes adigital communications channel to communicate the power available andthe power requests.
 3. The system of claim 2, wherein the digitalcommunications channel comprises a serial communications channel.
 4. Thesystem of claim 1, further comprising: a programmable current sense andcontrol unit coupled to power output for each port of the external powersupply.
 5. The system of claim 4, further comprising: a controller inthe external power supply configured to determine the allocation ofpower to the network switches; and a control bus coupling the controllerto each of the programmable current sense and control units in theexternal power supply.
 6. The system of claim 1, further comprising: aprogrammable current sense and control unit coupled to power output foreach port of each network switch.
 7. The system of claim 6, furthercomprising: a controller in each network switch configured to determinethe allocation of available power to the network devices; and a controlbus coupling the controller to each of the programmable current senseand control units in the network switch.
 8. The system of claim 1,further comprising: a power multiplexer in each network switch coupledto receive power both from the internal power supply of the networkswitch and from the external power supply, wherein the power multiplexeris controllable to switch available power to the ports of the networkswitch.
 9. The system of claim 8, wherein the power multiplexer iscontrollable to switch power from the internal power supply to a firstbank of ports, to a second bank of ports, or to both the first andsecond bank of ports.
 10. The system of claim 9, the power multiplexeris further controllable to switch power from the external power supplyto the first bank of ports, to the second bank of ports, or to both thefirst and second bank of ports.
 11. The system of claim 1, Wherein thepower requests are determined by arbitration between the networkswitches.
 12. The system of claim 11, wherein the arbitration comprisesa master-slave arbitration procedure.
 13. The system of claim 11,wherein the arbitration comprises a peer-to-peer arbitration procedure.14. A method of power distribution to network devices, the methodcomprising: determining amounts and priority levels of power for thenetwork devices connected to each power distributor of a plurality ofpower distributors; summing together the amounts at each priority levelat each power distributor; and determining additional amounts andpriority levels of power required beyond an internal power supplycapability of each power distributor.
 15. The method of claim 14,communicating a power request to an external power supply; andallocating power by the external power supply to the power distributorsdepending on the communicated power requests.
 16. The method of claim14, wherein the power distributors comprise network switches, andwherein the available power and power requests are communicated using aserial data connection between the external power supply and each powerdistributor.
 17. The method of claim 14, further comprising: arbitrationbetween the network switches to determine the power requests.
 18. Themethod of claim 17, wherein the arbitration comprises a master-slavearbitration procedure.
 19. The method of claim 17, wherein thearbitration comprises a peer-to-peer arbitration procedure.
 20. A methodof distributing power to network devices, the method comprising:associating an amount and priority level of power for each deviceconnected to a port of a network switch; maintaining in the switch atable of the amount and priority level for each switch port; and usingthe table to allocate available power to higher priority devices wheninsufficient power is available to fully power all of the connecteddevices.
 21. The method of claim 20, wherein the allocation of power iscontrolled using current control switches connected to the switch ports.22. The method of claim 21, wherein the allocation of power is furthercontrolled using a power multiplexer circuit which is configured toconnect to an internal power supply within the switch and to an externalpower supply.
 23. The method of claim 22, wherein the power multiplexercircuit is controllable to switch power from each said power supply to afirst bank of ports, to a second bank of ports, or to both the first andsecond bank of ports.
 24. The method of claim 21, further comprising:detecting actual power amounts drawn by the network devices usingcurrent sensors coupled to the switch ports.
 25. The method of claim 24,wherein if an actual power drawn from a specific port exceeds theauthorized power to that port, then the current switch associated withthe port is open to halt the power drawn therefrom.
 26. The method ofclaim 20, wherein the priority level of power for a switch port dependson which type of network device is connected to the port.
 27. The methodof claim 26, wherein the higher priority devices include IP-enabledtelephone devices.
 28. The method of claim 26, wherein the higherpriority devices include at least one wireless access point.
 29. Themethod of claim 20, wherein the priority levels of power for switchports are manually configurable into the table by a networkadministrator.
 30. The system of claim 1, wherein the external powersupply and the plurality of switches are integrated into a same unit.